Any unsaturated hydrocarbon with structure that is a result of the assembling of two isoprene units
Monoterpenes are a class of
terpenes that consist of two
isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen functionality or missing a methyl group, are called monoterpenoids. Monoterpenes and monoterpenoids are diverse. They have relevance to the pharmaceutical, cosmetic, agricultural, and food industries.[1]
Monoterpenes are derived biosynthetically from units of
isopentenyl pyrophosphate, which is formed from
acetyl-CoA via the intermediacy of
mevalonic acid in the
HMG-CoA reductase pathway. An alternative, unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP-(2-methyl-D-erythritol-4-phosphate) pathway, which is initiated from
C5 sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids).[2]
Elimination of the pyrophosphate group from geranyl pyrophosphate leads to the formation of acyclic monoterpenes such as
ocimene and the
myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid
geraniol. Additional rearrangements and oxidations provide compounds such as
citral,
citronellal,
citronellol,
linalool, and many others. Many monoterpenes found in marine organisms are
halogenated, such as
halomon.
Monoterpenes are found in many parts of different plants, such as
barks,
heartwood, bark and leaves of coniferous trees, in
vegetables,
fruits and
herbs.[5]Essential oils are very rich in monoterpenes. Several monoterpenes produced by trees, such as
linalool,
hinokitiol, and
ocimene have
fungicidal and
antibacterial activities and participate in wound healing.[6] Some of these compounds are produced to protect the trees from
insect attacks.
Monoterpenes are emitted by
forests and form
aerosols that are proposed to serve as
cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate.[7]
A study suggests that a range of
floor cleaners with certain monoterpenes may cause indoor
air pollution equivalent or exceeding the harm to respiratory tracts when the time is spent
near a busy road. This is due to
ozonolysis of monoterpenes like
Limonene leading to the production of atmospheric
SOA.[20][21] Another study suggests monoterpenes substantially affect ambient organic aerosol with uncertainties regarding environmental impacts.[22] In a review, scientists concluded that they hope that these "substances will be extensively studied and used in more and more in medicine".[23] A 2013 study found that "Based on adverse effects and risk assessments, d-limonene may be regarded as a safe ingredient. However, the potential occurrence of skin irritation necessitates regulation of this chemical as an ingredient in cosmetics."[24][better source needed] According to a review, several studies showed "that some monoterpenes (e.g., pulegone, menthofuran, camphor, and limonene) and sesquiterpenes (e.g., zederone, germacrone) exhibited liver toxicity" and that i.a. intensive research on terpenes toxicity is needed.[25]
^Davis, Edward M.; Croteau, Rodney (2000). "Cyclization enzymes in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes". Topics in Current Chemistry. 209: 53–95.
doi:
10.1007/3-540-48146-X_2.
ISBN978-3-540-66573-1.
^Sjöström, Eero (22 October 2013). "Chapter 5: Extractives". Wood Chemistry: Fundamentals and Applications (Second ed.). San Diego.
ISBN978-0-08-092589-9.{{
cite book}}: CS1 maint: location missing publisher (
link)
^Rowell, Roger M. (2013). "Chater 3: Cell Wall Chemistry". Handbook of Wood Chemistry and Wood Composites (2nd ed.). Boca Raton: Taylor & Francis.
ISBN9781439853801.
^Zhang, Haofei; Yee, Lindsay D.; Lee, Ben H.; Curtis, Michael P.; Worton, David R.; Isaacman-VanWertz, Gabriel; Offenberg, John H.; Lewandowski, Michael; Kleindienst, Tadeusz E.; Beaver, Melinda R.; Holder, Amara L.; Lonneman, William A.; Docherty, Kenneth S.; Jaoui, Mohammed; Pye, Havala O. T.; Hu, Weiwei; Day, Douglas A.; Campuzano-Jost, Pedro; Jimenez, Jose L.; Guo, Hongyu; Weber, Rodney J.; de Gouw, Joost; Koss, Abigail R.; Edgerton, Eric S.; Brune, William; Mohr, Claudia; Lopez-Hilfiker, Felipe D.; Lutz, Anna; Kreisberg, Nathan M.; Spielman, Steve R.; Hering, Susanne V.; Wilson, Kevin R.; Thornton, Joel A.; Goldstein, Allen H. (12 February 2018).
"Monoterpenes are the largest source of summertime organic aerosol in the southeastern United States". Proceedings of the National Academy of Sciences. 115 (9): 2038–2043.
Bibcode:
2018PNAS..115.2038Z.
doi:10.1073/pnas.1717513115.
ISSN0027-8424.
PMC5834703.
PMID29440409.
^Wojtunik‐Kulesza, Karolina A.; Kasprzak, Kamila; Oniszczuk, Tomasz; Oniszczuk, Anna (8 November 2019). "Natural Monoterpenes: Much More than Only a Scent". Chemistry & Biodiversity. 16 (12): e1900434.
doi:
10.1002/cbdv.201900434.
ISSN1612-1872.
PMID31587473.
S2CID203850556.
^Kim, Young Woo; Kim, Min Ji; Chung, Bu Young; Bang, Du Yeon; Lim, Seong Kwang; Choi, Seul Min; Lim, Duck Soo; Cho, Myung Chan; Yoon, Kyungsil; Kim, Hyung Sik; Kim, Kyu Bong; Kim, You Sun; Kwack, Seung Jun; Lee, Byung-Mu (1 January 2013). "Safety Evaluation And Risk Assessment Of d-Limonene". Journal of Toxicology and Environmental Health, Part B. 16 (1): 17–38.
doi:
10.1080/10937404.2013.769418.
ISSN1093-7404.
PMID23573938.
S2CID40274650.